Rectenna for high-voltage applications

ABSTRACT

An energy transfer system is disclosed. The system includes patch elements, shielding layers, and energy rectifying circuits. The patch elements receive and couple radio frequency energy. The shielding layer includes at least one opening that allows radio frequency energy to pass through. The openings are formed and positioned to receive the radio frequency energy and to minimize any re-radiating back toward the source of energy. The energy rectifying circuit includes a circuit for rectifying the radio frequency energy into dc energy. A plurality of energy rectifying circuits is arranged in an array to provide a sum of dc energy generated by the energy rectifying circuit.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefit of the priority of U.S. ProvisionalApplication Serial No. 60/138,302, filed Jun. 9, 1999, and entitledCompact, Dual-Polarized 8.51 GHz Rectenna for High Voltage (50 V)Actuator Applications.

ORIGIN OF INVENTION

The invention described herein was made in the performance of work undera NASA contract, and is subject to the provisions of Public Law 96-517(35 USC 202) in which the Contractor has elected to retain title.

U.S. Government may have certain rights in this invention pursuant toNASA contract number NAS7-1407.

BACKGROUND

The present disclosure generally relates to antennas that convert RFpower to DC power, and specifically to designs of such antennas used inhigh-voltage applications.

A rectenna is a special antenna that captures and converts RF ormicrowave power to DC power. The rectenna may be used as the receivingterminal of a power transmission system. In this configuration, theterminal may deliver DC power to a load where physical transmissionlines are not feasible. The power delivery may be through free space.The rectenna may also be useful in applications where DC power needs tobe distributed to a large number of load elements that are distributedspatially. The power distribution is achieved by the dispersive natureof microwave energy in space. The dispersion may substantially reducethe need for physical interconnects to individual load elements. Therectenna may use the dispersive nature of the microwave power to combinethe power from many elements, which are spatially separated by theelement spacing of the array or panel. Therefore, the effective area ofthe entire rectenna panel determines the total power received by thepanel.

An application for rectennas may involve the transmission of power toactuators. For example, the actuators may control the position ofindividual surfaces of a spacecraft-mounted optical reflector. The useof rectennas may simplify the design of the multi-surface reflector byeliminating the need for a wiring harness to distribute power to theindividual actuators. Further, the rectennas may also have associatedcircuitry to provide control signals to each actuator by propermodulation of the incident microwave beam. However, the actuators oftenrequire high voltage for operation. For example, the high voltage may beon the order of about 50 volts.

SUMMARY

The present disclosure defines an energy transfer system that includespatch elements, shielding layers, and energy rectifying circuits. Thepatch elements are arranged to face a source of radio frequency energy.Each of the patch elements has a first surface facing the source ofenergy, and a second surface opposite the first surface and facing awayfrom the source of energy.

Each shielding layer is located facing the second surface, such thateach of the patch elements is coupled between the source of energy andthe shielding layer. The shielding layer includes at least one openingthat allows radio frequency energy to pass through to a second sidethereof. The openings are formed and positioned to receive the radiofrequency energy and to minimize any re-radiating back toward the sourceof energy.

The energy rectifying circuit includes a circuit for rectifying theradio frequency energy into dc energy. At least a part of the energyrectifying circuit is on the second side of the shielding layer. Thecircuit is separated from the source of energy by the shielding layer.The energy rectifying circuits are arranged in an array to provide a sumof dc energy generated by the energy rectifying circuit.

The present disclosure also defines a method of receiving andtransducing energy. The method includes separating a first part of theenergy having a first characteristic, from a second part of the energyhaving a second characteristic, coupling the first part of the energy toa first circuit portion of an energy receiving board, and coupling thesecond part of the energy to a second circuit portion of the energyreceiving board. The second circuit portion is spaced apart from thefirst circuit portion. The method further includes appropriatelycoupling energy receiving boards to add energy received by each energyreceiving board.

BRIEF DESCRIPTION OF THE DRAWINGS

Different aspects of the disclosure will be described in reference tothe accompanying drawings wherein:

FIG. 1 shows panel construction of a microstrip patch antenna accordingto an embodiment;

FIG. 2A shows a microstrip rectifying circuit structure layout inaccordance with an embodiment;

FIG. 2B illustrates an array of rectennas extended to fill a large panelin accordance with an embodiment;

FIG. 3 shows a plot of scattering parameter data for a microstrip patchantenna;

FIG. 4 shows patch dimensions for a microstrip patch antenna;

FIG. 5 shows DC performance simulation data for M/A-Com 40401 Schottkydiodes;

FIG. 6 shows measured data for M/A-Com 40401 Schottky diodes;

FIG. 7 shows performance results of a typical vertical polarizationcircuit in microstrip patch antenna panels;

FIG. 8 shows performance results of a typical horizontal polarizationcircuit in microstrip patch antenna panels;

FIG. 9A illustrate a rectenna panel array layout in accordance with anembodiment;

FIGS. 9B and 9C tabulate results of effective area measurements for arectenna panel array;

FIG. 10 is a simplified circuit diagram of a rectenna panel arrayaccording an embodiment;

FIG. 11 shows measured results of the rectenna panel array when loadedfor optimum overall efficiency with a load resistance of 5.4 KΩ;

FIG. 12 shows measured results of the rectenna panel array with a loadresistance of 1 MΩ; and

FIG. 13 is a flowchart of a method for receiving and transducing energyin accordance with an embodiment.

DETAILED DESCRIPTION

The individual rectennas may be designed to provide the high voltage inan efficient manner. Trade-offs include physical rectenna size, diodeavailability, and diode characteristics including breakdown voltage andpower handling. Trade-offs also include efficiency of capturing themicrowave energy by the antenna, efficiency of the rectificationprocess, and frequency of the incident power. These parameters may alsobe dependent on each other. In one embodiment, the frequency of theincident microwave power was chosen to be 8.51 GHz. However, it shouldbe understood that while this design of 8.51 GHz was based on the RFsources available at the time, it may easily be adapted to differentfrequencies.

Previous rectenna designs have employed dipole antennas and received asingle linear polarization. A thin-film, printed-circuit dipole rectennadesign allowed simple DC removal. This single polarization design hasminimized the thermal path between the diodes and the outer surface.However, the printed capacitors of these thin-film designs haverestricted tunability of the capacitors.

For dual polarization, the extension of the thin film dipole design mayencounter substantial obstacles to successful implementation. A separatelayer may be required for each polarization. Electromagnetic interactionbetween the DC collection lines, parallel to the dipole on theorthogonal layer, may compromise the rectenna performance. Further,having one dipole layer “buried” beneath the other may also presentthermal problems.

The fabrication process of the dual linear dipole design may bechallenging since diodes and chip capacitors may be potentially buriedbetween layers of foam and/or polyimide film. The dipole rectennadesigns may also present disadvantageously couple the feed circuitry tothe dipole antenna. The twin lead transmission line impedance may berelated to the dipole impedance, thereby posing a constraint on thedesign. Also, the feed circuitry may be exposed to the outside world.The parasitic radiation from the feed lines including harmonic radiationmay present difficulties. The harmonic radiation may be generated by thediodes.

The inventors recognized that for dual-polarization needs, a microstrippatch design may alleviate many of the difficulties mentioned above.Advantages of dual-polarization include doubling the receive power perelement area, and making the rectenna capable of receiving either duallinear or a single circular polarization. The latter ability to receivecircular polarization makes the rectenna panel suitable for applicationssuch as circling airplane or space-born platforms including a space-bornoptical reflector.

An aperture-coupled microstrip patch 101 relies on an aperture, orcoupling slot 102, to couple electromagnetic energy from a feed circuit104, 105 to the microstrip patch antenna 101, as is shown in FIG. 1. Inthis configuration, the antenna 101 and the microstrip feed circuit 104,105 are isolated from one another. This may also allow the diodecircuitry to be located behind the ground plane 106 containing thecoupling apertures 102. The ground plane 106 separates the patch antenna101 and the feed circuit 104, 105, 106. This ground plane 106 mayprotect the feed circuitry 104, 105 from the incident RF energy. As aresult, the incident RF energy may not be coupled to the DC lines thatcollect the output power. The ground plane 106 may also reduce theharmonics from radiating back towards the incident wave. The harmonicsare generated by the diodes. A microstrip line filter may be used toprevent radiation back through the aperture feed 104, 105.

As shown in FIG. 1, the patch antenna 101 may be supported above theground plane 106 by a lightweight foam support 108. In this embodiment,the foam 108 has a relative dielectric constant of 1.07. The foam 108may reduce the possibility of surface wave modes that may limit themicrostrip patch array performance. The ground plane 106 may provide athermal sink for the diodes with via connections to the ground plane106.

In the illustrated embodiment of FIG. 1, the microstrip patch antennas100 includes Rohacell® 51 (ε_(r)=1.07), as a foam spacer 108 andRT/Duroid® 5880 (ε_(r)=2.2) as a microwave substrate 110. The microwavesubstrate 110 may be about 0.64 mm thick and have a copper thickness ofabout 17.5 μm. The patch antenna 100 may also include Sheldahl'sNovaclad® G2200 as a copper clad polyimide film 112 (ε_(r)=3.3), onwhich the microstrip patch antenna 101 may be etched. In thisembodiment, the polyimide film 112 is 50 μm thick and has a copperthickness of 35 μm. For every 25 μm change in the foam spacer'sthickness, a 10 MHz change in resonant frequency may occur. To insureaccurate thickness of the foam used, the thickness of the Rohacell® maybe pre-compressed to about 0.89 mm by using a compression fixture and anoven. A piece of about 1.02 mm thick Rohacell® foam may be placed in acompression fixture. The entire assembly may then be placed inside anoven that is heated above 191° C. At this temperature, the Rohacell® maylose its compressive strength. The use of about 0.89 mm shims maydetermine the final thickness.

Typical solder reflow temperatures may be above 205° C.; and theRohacell® may begin to expand at 191° C. Therefore, if the Rohacell® isbonded to the RT/Duroid® prior to the solder reflow process and thenexposed to the solder reflow, the foam may distort. The distortion ofthe foam may destroy the rectenna panel. An alternative solution mayinclude using a room temperature vulcanizing silicone rubber adhesive114, such as Dow Corning® 3140 RTV. Using the 3140 RTV Coating enablescomponents to be wave soldered to the RT/Duroid®. The RT/Duroid® maythen be bonded to the Rohacell foam. The fabrication process may apply auniform coat of adhesive, thereby insuring repeatability in thefabrication process.

FIG. 2A shows a layout of the microstrip rectifying circuitry structure200 according to an embodiment. However, it should be understood thatthe circuitry carrying out this function could easily be replaced by anyother well-known circuitry. The structure 200 includes the physicalrelationship between the circuitry, patch 202 and slots 204, 206. Slot204 overlies a first diode rectifying circuit 208. Slot 206 overlies theother diode rectifying circuit 210. In the illustrated embodiment, theterm “overlies” refers to a hypothetical plane that is perpendicular tothe plane of the layers and hence parallel to the direction of incidentmicrowave radiation. This plane may pass through both the slot and thecircuit portions. The design may be configured to use slots thatpreferably receive information from different energy polarizationcomponents.

The patch used can be of a rectangular, circular, oval, or ellipticalshape. For a single polarization, the slot is located down the center ofthe patch in an aperture-coupled patch system. The patch oftencompletely overlays the entire slot. However, the slot and the patch maybe placed relative to each other in such a way that allows at least oneof the slots 204, 206 to slightly overhang the edges of the patch. Thisimplementation of the aperture-coupled patch may be used for thisrectenna. The slight overhang can constitute 10 to 20 percent of thelength of the slot. This leaves a portion 216 of slot 206 overhangingthe area of the patch 202.

The energy may be coupled through slot 204 to microstrip feeder line220. Microstrip feeder 220 includes element 222 which terminates themicrostrip feeder to change its impedance and thereby obtain a bettercoupling action. Additional filter and impedance transformation elements224 may further process the signal to provide further filtering thereon,and to impedance transform for better operation. In this embodiment, thelow pass filtering operation filters the system to include signals offrequency less than 8.51 GHz.

A step area 226 may also be provided for additional matching. Thisadditional matching may also facilitate impedance matching relative todiode 228. The RF energy passes via microstrip waveguide 230 to diode228, which rectifies the RF to provide a DC signal near the area 230.The diode 228 may also provide more matching to the capacitor area 232that provides the additional DC conversion. The final DC-convertedsignal at 234 may be coupled to DC bus 212 or 214. The DC bus 212 or 214may be connected between the various patches to connect all of the DCfrom all the patches combined to their final destination. Each of thebus lines 212, 214 is connected to a plurality of the circuit elements208, 210 through the diodes by using an isolated ground plane under eachelement.

The patch rectenna may be implemented using standard microstrip lines.The use of a minimum number of stubs for the input filter/matchingsection 224 may enable the patch size to be decreased. The line lengthsmay also be made as short as possible to minimize the RF losses. Thecorresponding microstrip implementation of the vertical 208 and thehorizontal 210 polarization circuits is shown in FIG. 2A. The steppedline impedance 230 and chip capacitor 232 may perform the final steps ofbandpass filtering and matching.

FIG. 2B illustrates an array of rectennas extended to fill a largepanel. Each rectenna design is similar to the above design described inconnection with FIG. 2A.

The above microstrip patch design is configured to provide a low returnloss at approximately 8.51 GHz. The return loss for each port 104, 105is designated as S₁₁ and S₂₂, respectively. Further, the patch designenables an effective isolation between the two ports 104, 105 to providelow S₂₁ and S₁₂. The scattering parameter data and patch dimensions areshown in FIGS. 3 and 4. The measured return loss (S₁₁) 300 in FIG. 3 isshown to exceed −18 dB at approximately 8.51 GHz. Only port 1 is showndue to symmetry. The simulated return loss 302 is also shown.

The measured resonance frequency, for optimum return loss, was about8.565 GHz. The simulations indicate a worst case coupling 304 betweenthe two ports greater than −20 dB. Actual measurements were somewhatgreater than this value. Measurements show a worst case coupling 306between ports of −18.5 dB (at about 8.07 GHz) for all frequencies below10 GHz. To correct for the foam compression, the relative permittivitywas increased by the ratio of compressed to uncompressed height toε_(r)=1.2.

FIG. 3 also shows the predicted gain and scattering parameters up to 18GHz. This allows the simulated parameters to be shown at the firstharmonic of the operating frequency at 17.02 GHz. The measured resultsstop at 16 GHz where the microstrip line becomes over-moded. Thepredicted patch gain 308 at 8.51 GHz is 8.4 dB. At the second harmonicfrequency the gain 308 is less than −17 dB, indicating that the harmonicradiation is not radiated back through the coupling aperture and towardsthe signal source. The additional low pass filter, discussed below,provides further suppression of unwanted harmonic radiation.

Most rectenna designs have used a single diode in a clamping circuitconfiguration rather than a traditional multiple-diode rectifyingcircuit. At microwave frequencies, these rectenna circuits aresubstantially nonlinear and difficult to design based upon purelyanalytic equations.

Since optimal performance of a rectifier requires ideal tuning atharmonics as well as the fundamental frequency, parameters of any diodemodel must be known at the harmonic frequencies as well as at thefundamental frequency. Experimental large signal measurements provide amethod of extracting and verifying diode models and of searching formaximum efficiency.

For the purposes of the present disclosure, diodes readily available ina packaged format with large breakdown may be chosen. In one embodiment,the commercially available M/A-Com 40401 Schottky diode in package model213 is used. The rectification efficiency may be defined as$\begin{matrix}{{\eta_{r} = \frac{P_{D\quad C}}{P_{inc} - P_{ref}}},} & (1)\end{matrix}$

where P_(DC) is the DC output power, P_(inc) is the incident RF power,and P_(ref) is the reflected RF power. Overall efficiency may then bedefined by $\begin{matrix}{\eta_{ov} = {\frac{P_{D\quad C}}{P_{inc}}.}} & (2)\end{matrix}$

Measurement results at the design frequency of 8.51 GHz indicates amaximum overall efficiency of 66% with 65 mW of DC output power, andover 100 mW DC output power for a lower efficiency of 62%. This diodeexhibits a higher output voltage and higher efficiency than a similardiode by another manufacturer. The output voltage increased from 3.2 Vto 4.1 V at the lower efficiency of 62%, indicative of the trade-offbetween maximum output voltage and efficiency expected. The large outputvoltage of 4.1 V may allow for maximum output voltage if a suitablecombination method is used.

The diode is often the most critical component in the rectenna element.Many aspects of performance of the rectenna may depend primarily on thediode parameters. The series resistance, for example, may directly limitefficiency through the I²R loss. The junction capacitance, together withpackage capacitance and lead inductance, may affect how harmoniccurrents oscillate through the diode. The breakdown voltage may limitthe power handling capability of each rectifying circuit. Theseparameters may also affect the match of the circuit. Since the diode, asa power-rectifying element, operate in a large signal environment, thediode model may need to be valid for a wide range of biasing. Since highefficiency requires proper termination of harmonic frequencies, themodel may also need to be valid over a wide frequency range.

The simulation data for the DC performance is shown in FIG. 5. Thevoltage, current, and resistance data were taken from the IV curve, andthe total capacitance was measured by resonating the diode with a seriesinductor. The simulation data shows that the diode model agrees wellwith the measured data shown in FIG. 6. The breakdown voltage wasselected to be slightly higher since it was possible to select diodes,with breakdown around 9.5 V.

The parameters Vf1, Vf2, and Vf3 represent the forward voltages of theselected diode for the current stimuli of 0.01 mA, 0.1 mA, and 1.0 mA,respectively. The parameter Ir1 is the reverse current for the voltagestimulus of 1.0 V. The parameter Ct is the total capacitance. Vb is thebreakdown voltage. Rs1 and Rs2 represent the series resistance when thecurrents are as listed.

In some embodiments, measurements of the unit cells 240-246 shown inFIG. 2A may be formed without the microstrip patch and DC collection tothe next unit cell(s). The device under test (DUT) may include a singleunit cell 240, 242, 244, or 246 with a circuit for horizontalpolarization 210 and a circuit for vertical polarization 208 in the samelayout, as they would appear in an array. In other embodiments, theinput microstrip lines may be extended to the edge of the substrate tocoaxial connectors. DC output may be picked off the circuit by solderinga single-strand wire directly to the DC bus line 212, 214. A decade boxmay be used as the load.

Several unit cells were fabricated and measured. FIGS. 7 and 8 show themeasured results for three different boards. The rectificationefficiency of all circuits remained close to the design overallefficiency of 60%, with high overall efficiency when reflected power wasminimized. The average output voltage of the horizontal polarizationcircuits was 4.14 V at an average overall efficiency of 57.7% when usinga load resistance of 325 Ω. This desirable result meets the maximumoutput voltage from the diode measurements.

The average output voltage of the vertical polarization circuits waslower, 3.84 V, at an average efficiency of 49.8% with a load resistanceof 325 Ω. The trade-off of the higher output voltage for efficiency isindicated by the lower average output voltage of 3.56 V for the higheraverage efficiency, 53.7%, when a load resistance of 250 Ω is used.Higher sensitivity to variations in component assembly may havecontributed to the lower average overall efficiency for the verticalpolarization circuits.

Rectenna panels with a 3 by 3 arrangement of unit cells may befabricated using the above-described design. Since the average outputvoltage of the circuits is 4 V, the series connection of 13 circuits maysatisfy the design goal of 50 V. Since each patch provides two circuits,one for vertical polarization and another for horizontal polarization,the minimum number of cells required is 7 patch elements. Choosing theminimal square array containing at least 7 patches leads to an array of3 elements by 3 elements, for 9 total patch elements.

The effective area of a single patch element is given by,$\begin{matrix}{{A_{p} = {G_{p}\frac{\lambda^{2}}{4\quad \pi}}},} & (3)\end{matrix}$

Where G_(p) is the gain of the patch, or 8.4 dB at 8.51 GHz. Theeffective area for a single patch, before placement in the array, istherefore 6.8 cm² from equation (3). In order for the array to absorbsubstantial portion of the incident power, it may be necessary for theunit cell area be less than 6.8 cm². For rectangular spacing, a minimalcell-to-cell spacing of 2.62 cm may be required. Accordingly, a moredense cell-to-cell spacing of 1.97 cm may be used to further shrink theoverall panel size while still leaving sufficient room for the panelcircuitry.

During the diode and unit cell measurements, the overall efficiency maybe defined to be the total DC output power at the load in proportion tothe incident power. Thus any power that is reflected by the circuitrymay not be converted into DC power and may decrease the overallefficiency. Likewise in the panel measurement, overall efficiency may bedefined such that any reflected power from the panel lowers the overallefficiency. One method of computing overall efficiency for a rectennapanel may be to use the effective area of the panel.

Using the effective area of the panel, the received power is given by,$\begin{matrix}{{P_{recv} = \frac{P_{trans}G_{trans}A^{eff}}{4\quad \pi \quad R^{2}}},} & (4)\end{matrix}$

where A^(eff) represents the maximum effective area of the panel as thesum of the maximum effective area of each patch in the arrayconfiguration. Thus it may be seen that for rectenna panel applicationsit is desirable for the maximum effective area of the panel to exceedthe physical area of the panel. The maximum effective area of each patchshould be measured in the array configuration under matched conditions,i.e. with no reflection, when mutual coupling is present.

If the overall panel efficiency is then defined as, $\begin{matrix}{{\eta_{ov} = {\frac{P_{D\quad C}}{P_{recv}} = \frac{4\quad \pi \quad R^{2}P_{D\quad C}}{P_{trans}G_{trans}A^{eff}}}},} & (5)\end{matrix}$

any power reflected from the rectenna panel will provide a decrease inthe overall efficiency.

To properly compute the maximum effective area of each patch in thearray configuration, a rectenna panel may be built where a microstripline leading to a SMA connector replaces each rectenna circuitry asshown in FIG. 9A. Each port may be individually tuned until the returnloss for all patches exceeds 21 dB with all other ports matched. Onlyone polarization may be measured and the panel may be is rotated for theorthogonal polarization because of the symmetry. Thus only one port isshown in FIG. 9A.

The line losses and connector losses may be removed by calibrationstandards for each of the two microstrip feed line configurations. Toensure accuracy of the effective area measurements for the tightlypacked array, the mutual coupling between ports may be measured. In thisconfiguration, the coupling was measured to be less than 17.5 dB for allports. The low mutual coupling for this densely packed array may beattributed to the use of the low dielectric foam superstrate.

The results of the effective area measurements are shown in FIGS. 9B and9C. Using symmetry to reduce measurement error, the effective area maybe averaged between the two measurements representing the two orthogonalports of each patch. The effective area of the center patch closelymatches the unit cell area, as expected. Further, the effective area ofthe corner patches may be slightly larger than the unit cell area, sincethese patches are on the outside of the array.

In order to minimize the measurement discrepancies in the panelmeasurements that follow, the corner and center-edge effective areameasurements may be averaged using a symmetry argument. The totaleffective area of the 3 by 3 panel may be about 1 cm more than thephysical area of the panel, or 3% greater than the physical area. Thephysical area of the panel may be defined as nine times the unit cellarea.

A series combination of all individual rectenna circuits may be desiredas shown in FIG. 10. The combination may maximize the output voltage ofthe panel. To achieve this combination, the ground plane around eachindividual patch may be DC isolated below each patch. The ground planemay contain the coupling apertures. The DC isolation may be provided byetching a square ring slot 250 (see FIG. 2A) in the ground plane beloweach individual patch. To ensure RF continuity of the ground plane, andtherefore re-radiation of harmonics generated by the circuitry throughthese ground plane slots, a thin layer of copper-coated polyimide may beused to ensure capacitive coupling.

In the illustrated embodiment of FIG. 10, the diodes are installed in anopposing manner. This design may allow for a series output voltage foreach patch. In order to properly reverse bias all diodes when power isapplied, additional resistance r₁ through r₆ between the isolated groundplanes may be needed. In this embodiment, the resistance value for eachof the resistors r₁ through r₆ is chosen to be 1 MΩ. The additionalresistance may also allow each isolated ground plane to discharge whenpower is removed, protecting the diodes. The series connection of diodesD₁ through D₁₈ provides desired high voltage by linking the voltage ofeach diode, which may represent a bank or plurality of diodes.

FIG. 11 shows the measured results of the rectenna panel when loaded foroptimum overall efficiency with a load resistance R (in FIG. 10) equalto 5.4 KΩ. The overall efficiency may be calculated using equation (5).The load resistance of the entire panel may be about 18 times the unitcell resistance. The series connection of the circuits on the panel mayinvolve connecting slightly different circuits. Therefore, a startingload resistance of approximately 18 times 325 Ω or 5.85 KΩ may be closeto giving optimal overall panel efficiency.

The overall panel efficiency exceeds 52% over a large region of inputpowers, with a peak of 53% at a receive power of 38.8 mW/cm². Thedesired output voltage of 50 V may be achieved for an input powerdensity of 25.2 mW/cm². For the Narda 640 standard gain horn, thisrequires a transmit power of 13.6 W at a distance of 37.1 cm to provide50 V of output power. To provide maximum efficiency, the panel mayrequire not only sufficient loading, but also sufficient input power toplace the diodes in an efficient region of operation. The peak overallpanel efficiency is 4% less than the average efficiency of thehorizontal polarization unit cell measurements and 1% less than theaverage vertical polarization unit cell efficiency (measured without thepatch via microstrip line). This indicates that additional gains inefficiency are most likely to be obtained by optimization of the unitcell circuitry and not from further antenna optimization.

FIG. 12 shows one embodiment of the present disclosure having a loadresistance R (in FIG. 10) equal to 100 KΩ. By increasing the loadresistance, the required 50 V output may be obtained for a lowerincident power density of 6.3 mW/cm². This corresponds to only 3.4 W oftransmit power at a distance of 37.1 cm.

Since the expected output voltage of 18 diodes in series could exceedthe desired output voltage, a method of producing more than one 50 Voutput may be desired. By attaching commercially available boostregulator circuits it may be possible to obtain two 50 V outputs. Theseries output of 5 patches, or 10 circuits, may be used to drive oneregulator and the remainder to drive the additional regulator.Therefore, the boost regulator circuits may lower the required incidentpower density even further.

A flowchart of a method for receiving and transducing energy inaccordance with an embodiment is shown in FIG. 13. At 1300, a first partof the energy having a first characteristic is separated from a secondpart of the energy having a second characteristic. The first part of theenergy is coupled to a first circuit portion of an energy receivingboard at 1302. The second part of the energy is coupled to a secondcircuit portion of the energy receiving board at 1304. The secondcircuit portion is spaced apart from the first circuit portion.

An RF shielding layer is inserted at 1306, to shield the first andsecond circuit portions from RF energy. The RF shielding layers are dcisolated at 1308, and coupled to each other with a plurality ofresistors at 1310. The energy receiving boards are appropriately coupledat 1312. The boards are coupled by serially connecting a plurality ofthe first and second circuit portions in the energy receiving boards.

While specific embodiments of the invention have been illustrated anddescribed, other embodiments and variations are possible. For example,even though the array panel size has been described in terms of a 3 by 3array, array panels of different sizes may be used to achieve differentvoltage levels.

All these are intended to be encompassed by the following claims.

What is claimed is:
 1. An energy transfer system, comprising: antennaelements arranged to face a source of energy, said energy coupledthereto, each of said antenna elements having a first surface facing thesource of energy, and a second surface opposite said first surface andfacing away from the source of energy; a plurality of shielding layers,each shielding layer located facing said second surface, such that eachof said antenna elements is coupled between said source of energy andsaid each shielding layer, said each shielding layer including first andsecond apertures arranged to receive the energy, said first and secondapertures operating to separate a first part of the energy having afirst polarization from a second part of the energy having a secondpolarization, where said first aperture is positioned to receive saidfirst part of the energy, and said second aperture is positioned toreceive said second part of the energy; a plurality of energy receivingboards, each board including first and second circuit portions spacedapart from each other, said first circuit portion including a firstdiode and operative to couple said first part of the energy to produce afirst output signal corresponding to said first polarization, saidsecond circuit portion including a second diode and operative to couplesaid second part of the energy to produce a second output signalcorresponding to said second polarization, said second circuit portionusing same antenna element as said first circuit portion to couple theenergy; and means for appropriately coupling said first and secondoutput signals to sum the energy received by said each board of saidplurality of energy receiving boards; and a plurality of resistors, eachresistor coupled between each shielding layer, said plurality ofresistors arranged to appropriately reverse bias said first and seconddiodes when the energy is being received, and said plurality ofresistors are arranged to provide a discharging path to protect saidfirst and second diodes when the energy is removed, wherein said firstand second output signals of each antenna element are connected in aseries connection or a parallel connection such that said antennaelements are connected.
 2. The system of claim 1, wherein said eachshielding layer is a dc isolated ground plane.
 3. The system of claim 2,wherein said means for appropriately coupling said first and secondsignals includes a series dc path provided by said dc isolated groundplane operating to combine said first and second energy polarizations ofeach antenna element by summing said first and second signals, and bysaid plurality of resistors arranged to provide appropriate signalratios between said dc isolated ground planes.
 4. The system of claim 3,wherein said plurality of resistors are arranged to provide appropriatesignal ratios between antenna elements when energy from each antennaelement is combined with a next antenna element.
 5. The system of claim2, wherein said dc isolation is provided by a square ring slot etched onsaid each shielding layer.
 6. The system of claim 2, wherein said firstand second diodes are connected to said each shielding layer or dcisolated ground plane through a via connection.
 7. The system of claim1, wherein each of said plurality of resistors are connected betweensaid plurality of shielding layers, where said plurality of resistorsprovide further isolation of said plurality of shielding layers andprovide static discharge protection between said plurality of shieldinglayers.
 8. The system of claim 1, wherein each of said first and secondapertures includes a rectangular slot having a length along an axialdirection, said axial direction of said first aperture facing in adifferent direction than said axial direction of said second aperture,said each aperture receiving a different portion of energy inconjunction with each antenna element, where energy from said firstaperture is coupled to said first diode, and energy from said secondaperture is coupled to said second diode.
 9. The system of claim 1,wherein said antenna element is a patch antenna element.
 10. The systemof claim 1, wherein said antenna elements are arranged as an array insuch a configuration as to minimize a total effective area of the systemand increase overall system efficiency.
 11. The system of claim 1,further comprising: a plurality of surface wave reducing layers, eachsurface wave reducing layer coupled between each of said antennaelements and said each shielding layer.
 12. The system of claim 11,wherein said each surface wave reducing layer is formed of alow-dielectric lightweight material.
 13. A method of receiving andtransducing energy, comprising: separating a first part of the energyhaving a first polarization from a second part of the energy having asecond polarization in a first cell; coupling said first part of theenergy to a first circuit portion of an energy receiving board toproduce a first output signal; coupling said second part of the energyto a second circuit portion of the energy receiving board to produce asecond output signal, said second circuit portion spaced apart from saidfirst circuit portion; appropriately coupling energy of said first andsecond output signals to sum the energy received by said energyreceiving board; and providing dc isolation to said first cell.
 14. Themethod of claim 13, further comprising: inserting a shielding layer toshield said first and second circuit portions from RF energy.
 15. Amethod of receiving and transducing energy, comprising: separating afirst part of the energy having a first polarization from a second partof the energy having a second polarization; coupling said first part ofthe energy to a first circuit portion of an energy receiving board toproduce a first output signal; coupling said second part of the energyto a second circuit portion of the energy receiving board to produce asecond output signal, said second circuit portion spaced apart from saidfirst circuit portion; appropriately coupling energy of said first andsecond output signals to sum the energy received by said energyreceiving board; inserting a shielding layer to shield said first andsecond circuit portions from RF energy; isolating a plurality ofshielding layers; and coupling said plurality of shielding layers with aplurality of resistors.
 16. The method of claim 15, wherein saidappropriately coupling energy includes making serial or parallelconnection between each of a plurality of first and second circuitportions in said energy receiving boards.
 17. The method of claim 15,further comprising: providing said energy to a load; and transducingactuators.
 18. A circuit for coupling and receiving RF energy andconverting the RF energy to rectified dc energy, comprising: an array ofenergy receiving elements arranged to face a source of RF energy; aplurality of rectifying circuits operating to convert the RF energy intostable dc energy, each rectifying circuit including at least two diodesarranged in an opposing manner, said each rectifying circuit seriallyconnected to adjacent rectifying circuits to sum energy coupled in saidat least two diodes; and a plurality of ground planes, each ground planecoupled between each energy receiving element and said each rectifyingcircuit to minimize any re-radiating back toward the source of RFenergy, said each ground plane dc isolated and connected to adjacentground planes through resistors.